I had intended to turn this in to an article, or just post it on my blog. However, I came up short on content, and thought it would better serve the community here. The forums also provide more exposure, and thus a higher potential for interaction and feedback. I know others (Ryan J., Dave C., etc.) have likely had similar experiences and will hopefully chime in.

In the late fall of 2007 I was roughly 175 pounds and in the range of 18-20% body fat.

This photo is from 3-day loop around Newton Bald in the Smokies. We experienced temperatures in the low teens, and I was toasty in a thin wool shirt and 50g high loft synthetic jacket.

Between the first of 2009 and 2010 I dropped to around 138 pounds and a pretty consistent 6-7% body fat. I’ve since been out in similar conditions to that 2007 trip, and despite a couple layers of down on my torso (probably 3x the warmth of that 50g jacket I had), I was cold.

On a recent trip, we experienced pouring rain and 60-65 degree temps for a couple hours. My wife and dog enjoyed themselves in the cooling rain, a nice break from the southern heat, while I was miserable. Even at a light jogging pace I was covered in chill bumps and trying not to shiver.

The other side of this is thermodynamics. Post meal, when turning in to bed, or during physical activity (especially of the intense variety) I give off heat like a furnace. But, as soon as I stop moving or run out of calories, I chill quickly.

This photo is from a traverse in the Bob Marshall Wilderness Complex of Montana. I was wearing a thin merino wool hoody and compression shorts, as well as full gaiters and snow shoes. But, if we stopped moving for very long, I started to get cold and had to suggest we start moving again.

So what does all this really mean? A tipping point with body fat appears to exist, where going too low requires carrying additional insulation to maintain warmth. Finding that exact point would require study of a more scientific nature, but I’d estimate it to be in the range of 10-12%. Below 10% and you start to need additional external insulation to make up for a lack of internal insulation. Were that the only trade-off, I’d probably say it’s a good one. After all, a one pound down jacket is, in theory, likely to insulate the core far better than a pound of fat would. Unfortunately, low body fat also starts to limit your reserves, thereby limiting your potential trip length. We can only carry so much food into the back country, and it’s never quite enough to cover our daily expenditure. Couple that with little to no on-board reserves and you spell trouble, fast.

I'm sorta of in the same boat. I'm 5' 8" and now around 139 lbs, after following the Paleo Diet since January, and changing my exercise strategy. I've lost fat from places I didn't know I had fat. I'm really re-thinking my backpacking diet — more protein, more fat.

That said, I really enjoyed my last trip without the 20 lbs of excess body fat…

…as well as full gaiters and snow shoes. But, if we stopped moving for very long, I started to get cold and had to suggest we start moving again

Read something similar a few years from BP or Outside, where mountaineers stated they added body fat for high altitude mountaineering. Probably more for energy stores but could get into insulation. Snowshoeing season coincides with Thanksgiving and Xmas meals, so haven't had to worry about this for day-trips.

Makes sense that very low fat reserves could make your body think it's in danger of starvation, so it will protect itself by ramping down your metabolic rate. That could contribute to loss of cold tolerance at least as much as the lost insulation.

Will – that might be possible if you get in to essential fat, but I've never heard of it occurring. Quite the opposite is true in my case – I burn through calories at an insane rate. The only way I've been able to gain weight in any recent period was by eating 6-7k calories a day while being mostly sedentary.

Well, I'm certainly no scientist but I can share my personal experiences here and you may be dealing with something more than just body fat percentage. What exactly it is, I'm not sure. (Sorry for this long post but maybe something will help.)

I've been at about 4.5% body fat at 160 lbs. and I'm now probably around 20% body fat at 180 lbs. The one thing I can tell you is that at both high and low body fat percentages, there seems to be some magical temperature where I get cold when I'm outdoors and not moving.

I sweat like a crazy man when I'm on the move. I heat up quickly and stay that way until I stop. Then I seem to chill quickly and find it hard to recover if I don't properly insulate at the right time. My internal temperature swings seem to be greater than many of the people I've hiked and camped with and I'm thoroughly convinced it's a combination of variables that I haven't fully identified yet or have failed effectively control.

Generally, I seem to require more insulation in and around camp after activity has stopped. The same holds true for my sleeping bag and pad. NeoAir below 40 degrees F? Maybe even down to 35F? Maybe for many people but not for me! Been there, tried that and failed.

It's taken me a few years but I'm slowly getting better at dealing with it. Even though I haven't really identified what "it" is that makes me feel cold. Probably a combination of many things including: body fat %, age, hyrdation levels, etc.

One thing I am sure of is that someone will probably weigh in on this topic who really knows some things and give some good advice. Excellent topic and good luck!

I know from following your blog over the year or so that you do lots of functional strength training, body weight exercises, etc. What would happen if you increased resistance into your workouts and tweaked your diet without spinning off routine too far, could this help with your warmth issues on trips?

BTW, I think your transformation from the old Chris to the leaner Chris is pretty killer, especially how you've maintained it so well.

You mention requiring 6-7K calories to gain any weight, even while sedentary. This is a bit unusual, to say the least. It sounds like you might have an elevated basal metabolism rate, cause unknown. Have you considered seeing an endocrinologist?
I'd almost bet one of the first things he/she would do is order a BMR test. It could be as simple as your intense training schedule, or it could be something hormonal. Just a thought, since the other bases seem to be well covered by other posters.

Chris, are you eating many carbs? If you're on a paleo diet, with very low amounts of carbs, it could very well be that you are exercising beyond the ability of ketones to deal with the energy needs. With a lot of strenuous exercise, you need carbs.

I'm firmly with Mark Sisson (of Primal Blueprint) on the idea of keeping exercise to a moderate level. One of the reasons I disagree with Crossfit (I did it for two years), is that it asks far too much from what normal animal bodies do. There is something inherently wrong with doing a circuit that routinely has you getting close to vomiting… that simply isn't the way our bodies were meant to be used (otherwise the urge to vomit wouldn't appear). You will never see wild animals, including our human ancestors, doing anything like that. And yet they are/ were very strong and fit.

Perhaps you are overdoing it? Just an idea, since I'm certainly no expert on this. When I first read your post, one of the first thoughts I had was that it seemed very similar to anorexia.

My personal goal is to put on about 15 lbs with 10 being muscle. That should put me around 155 and 10% fat, all while not requiring me to buy new clothes. :-)

Eugene – I'm not sure just adding lean muscle would help much. In fact, it might be counterproductive as I'd have to eat even more than I do now to maintain any type of warmth. I'm definitely no expert though.

Tom – My wife has also mentioned my metabolism being so jacked up that I burn off pretty much anything I eat. I haven't had any tests done but it may be worth checking in to once I have insurance again. I'm far less active than when I was trying to lighten up my body and increase my fitness level, but my metabolism seems to still be around the same rate. I even tried hitting the gym for a year while modifying my diet to increase calories and protein. Nothing came out of it. The weight gain I mentioned occurred over a 1 month period and equated to about 3 lbs. I added a 2000 cal shake twice a day to my normal diet and only hit the gym 1 day per week for about 30 mins. I also didn't run or bike at all during that period. As soon as I went back to my normal routine, I promptly lost it all and a bit more.

All of the experimentation can be fun, but also expensive and quite the daunting task.

Chris – My guess is that you'll get it dialed in sooner rather than later, now that you're starting to really focus on it. One thing for sure is that you'll be a much happier camper at 10% body fat, and it will definitely extend your range on trips. That is quite a journey you've been on. Best of luck!

This is a scary topic for me. The most I ever weighed was 175 and that was about 6 years ago. Now I average 145 and have no clue what my body fat % is; a 29" waist may tell you something.

My problem is, certain health related issues require me to eat a low carbohydrate diet with very little sugar. Eating high calorie foods low in carbs and weight is a challenge. Cycling and hiking regularly now does not help me keep any weight on either.

I really want to do a thru-hike but not being able to put on any weight and having no fat reserves is starting to scare me. I would have to consume 5000+ calories a day but it has to be low in carbs and that may be very difficult and HEAVY.

Like others here, I tend to get cold when not active and sleep somewhat cold. My NeoAir and WM Summerlite are only good to about 40 degrees. Anything below that and I use a better pad and 15 degree bag.

Just want to point out that Chris' version of sedentary is a far cry from what most might quantify as such. He really can't sit still for long. ;p

I had the same issue a few years ago, in fact I couldn't stop losing weight even though I ate 4 or 5 huge meals a day, everyone told me I needed to gain weight, including my doctor, but I felt I was in the best shape of my life.

Chris's metabolism is definitely jacked, but regardless of whatever quantification a physician would put to it, it's a natural reaction to the sheer amount of activity he sustains on average. When I was working outside full time, exercising, and participating in strenuous sports, it was the same. I don't think your body can run full-tilt 90% of the time and then just throttle back.

In fact, even after my lifestyle became much slower, with orders of magnitude less exertion, my metabolism stayed elevated, for well over a year. Only now after continued loafing have I managed to reach a relatively normal weight. Which btw, feels fat to me, but according to my doctors is in line with where I should be. Regardless, I know I'd be feeling healthier 5 lbs of beer lighter. ;)

Ultimately, what I think Chris is really getting at here, and starting to really understanding himself, is that there's definitely a line you can cross. Just as it can be hard to stay healthy in an overly sedentary lifestyle, it can be with an overly active one also.

The important thing is that he's acknowledging that, where as many people I see that become obsessed with activity and fitness, never realized when they've crossed that barrier. I don't think our culture really allows us to believe that you can be too "skinny" unless you've got an eating disorder, but you can, and as Chris is seeing, it can have consequences. It's especially compounded by the fact that's he's not "frail" at all, he's very muscular, however, most people with his physique get it from a gym, and only put it to test in a gym.

Peak physical "strength"(endurance, fitness, whatever) and physique are not synonymous, even if they often line up.

Don't worry Chris, if we keep with the Tequila and post hike Waffle House diet, it's bound to take it's toll eventually, even if we do 32miles in 12 hours.. =”,jdempsey”

Piper and Mary: By no means was I trying to offend anyone. I just feel for the OP and can relate.

From what I have been told by male thru-hikers, some women fair much better on the long distance treks than men do; something about how the difference in metabolisms allows the gals to remain overall healthier than the guys. I am sometimes envious of your side for these reasons! And you hate us? : )

To those of you who find it hard to lose weight… please remember that Chris got to where he is with a lot of exceptionally hard work, plus a lot of study of nutrition. You may find that the paleo-diet will work for you, too. It's based on a lot of research done on how our ancestors before agriculture (when we lived in our natural genetic state for our species) lived and ate. Three books that will give you a lot of insight into how it works, "The Primal Blueprint" by Mark Sisson (it totally changed my life as a diabetic, and it is more of a healthy lifestyle guide than purely book on nutrition. Does a great job in conjunction with Sisson's very active online community and blog, http://marksdailyapple.com), "Good Calories, Bad Calories" by Gary Taubes (presents the political and scientific fiasco that hid the truth about how fat was vilified and carbohydrates championed… and goes on to argue how our bodies were never meant to eat so many carbohydrates. Extremely well-researched and argued book), and "The Paleo Diet", by Loren Cordain (researches the way our Paleolithic ancestors ate and how we can replicate it today. A little too macho for my taste with a much too simplistic understanding of how hunter gatherers must have lived day-to-day, in great part because it is so painfully obvious the the writer knows almost nothing of the complexity and richness of other cultures and how they think and live, but a good insight into the kind of food that is healthiest for us.

Going Primal, with low amounts of carbohydrates (no grains, very little sugar, small amounts of fruit), very high consumption of vegetables, quite high amounts of healthy fats (no vegetable oils or margarine), and moderate amounts of protein has made me lose weight naturally and steadily. Soon I'll be back to my college weight of 70 kg. That and steady exercise (lots of walking) with occasional bodyweight strength exercises and sprints is really working. You might as well try. If nothing else, cut back on all grains and sugar. That will do wonders.

My experience is similar to Chris' insofar as finding it really hard to stay warm when lean goes. I've never had too crazy a metabolism though, 3500 calories seems to keep me even these days (thankfully).

My wife is quite lean for woman, and that combined with her sedate metabolism makes our experiences staying warm outdoors strikingly different. Not only does she need more clothing for given temp, but doesn't create almost instant heat when working hard in that way I do. Certainly some gender/biological differences at play. Especially insofar as the latter point is concerned I wouldn't want to be in her shoes.

On my long distance PCT hikes I got down to about 120lbs (I'm 5'3"), which wouldn't be considered emaciated or even thin. Then I slowly gained some of it back on the trail even though I hiked 25+ mile days. The female body, even one past its reproductive years, knows how to keep the weight on.

I do feel bad for you guys who can't keep any weight on despite eating a lot of food. Eating tons of food all the time, like I did as a long distance hiker, got really old after a while.

I'm basically 5'8 (just a hair under). At 175ish I had already lost 45 lbs primarily through diet. I had just started to get back in to backpacking, hiking, and mountain biking around that time. Shortly after I started doing light runs and got down to around 163. Then I did P90X a couple of times along with an increase in the running and hit where I'm at now.